Tax inversions — deals in which a U.S. company gains the ability to domicile in a foreign country and trim its U.S. tax burden — are frowned upon by politicians and the Treasury Department. But efforts to penalize companies for making such deals seem to be falling flat.

The Pfizer deal announced Monday will push the total value of tax inversions this year to new highs. The number of such moves announced this year is also higher than was normal prior to 2014, according to data compiled by Dealogic.

The high price Pfizer offered for Allergan was likely boosted by the tax benefits of redomiciling in Ireland, a move that could cut the company's adjusted tax rate by 8 percentage points. Pfizer said the deal would put it "on a more competitive footing within our industry."

The pharmaceutical industry has been especially quick to take advantage of inversion deals. Allergan itself was purchased in another big deal by Actavis last year (which took the purchased company's name), and others like Pozen and Questcor Pharmaceuticals also have taken advantage of inversion deals. Pfizer tried a similar move last year in a bid to buy U.K.-based AstraZeneca.

Tax inversions are a better deal for companies that already have substantial sales abroad, or (like pharmaceuticals) rely on high-margin businesses based on intellectual property.

But while inversions have continued at an unusually high rate, overall merger and acquisition activity has been slowing. While the $4.2 trillion value of global M&A deals for 2015 has broken the 2007 record (according to Thomson Reuters), much of that value is simply a reflection of the high valuation of companies overall and a few huge deals.

Removing the effect of those high valuations and simply looking at the number of deals going through makes the numbers far less impressive. M&A deals this year have been driven by the largest transactions. This year is set to have fewer deals than most others since 2007.

In terms of the number of deals, last month was the weakest October for M&A in almost 20 years. Huge acquisitions like InBev's proposed purchase of SABMiller brought the dollar value to about half a trillion, but the total of smaller deals fell to fewer than 3,000.

A recent survey by M&A law firm Dykema found that professionals are starting to doubt the strength of the deal market after years of runaway activity. About 20 percent of respondents said they think the market will weaken in 2016, compared to only 9 percent last year. About 37 percent said they expect it to be strong, compared to 59 percent a year ago.

By value, U.S. inversion deals accounted for about 4 percent of global M&A activity in 2014 and 2015 — up from 2 percent or less in the three years before that.

It remains to be seen whether Treasury Department rules meant to prevent tax inversions will have an impact. The regulations do not apply to the Pfizer deal, which will result in a company with about 56 percent ownership in the U.S. — the rules only apply to companies with 60 percent or higher.

Politicians likely will have a lot to say about the inversion controversy heading into the 2016 presidential election.

Clarification: This story was updated to delete the reference to Abbott Laboratories taking advantage of an inversion deal. Abbott sold its overseas branded generics business to Mylan, which re-domiciled in the Netherlands. Abbott Laboratories is still headquartered in the U.S.